Armor works normally (give AC bonus) until the character goes to 0 hp then Armor absorb excess damage from that hit.

Note: This damage reduction works only on the hit that would take the character below 0hp, if the character is already out of combat the damage reduction doesn't work, so if attacked again you apply the normal rules against helpless target.

The Damage Reduction given by armors will be equal to their armor bonus. Let's call this damage reduction (DR) a DR at Zero hp (or DRZhp).

Example: Rodrick a 3rd level fighter with 21 hp and a chain mail has +5 bonus to AC (and so also 5 DRZhp), in a long fight is reduced to 4 hp, then he is it for another 8 points of damage. Normally this would take Rodrick to -4 hp and on his way to Walhalla, but with this rule Rodrick will go to 0 hp since the additional 4 hp of damage will be absorbed by his chain mail.